The invention relates to an electrical switchgear apparatus, in particular a current limiting apparatus such as a current limiting circuit breaker, the external manifestations whereof when breaking is performed are reduced or even non-existent.
In the document FR 2,589,624 a conventional arc extinguishing chamber for a low-voltage power circuit breaker is described. The chamber is situated facing the separable contacts of the circuit breaker and is provided with gas outlet orifices on a rear wall opposite from the contacts. Flat metal fins are arranged inside the chamber, between the contacts and the gas outlet orifice, perpendicularly to the side walls. In the event of a short-circuit occurring, separation of the contacts gives rise to an electric arc which is projected into the chamber by an electromagnetic loop effect. As it progresses in the chamber, the arc encounters the fins which absorb a part of its energy. The arc also exchanges heat with the side walls of the chamber which are made of synthetic gas-generating material. The arc cools progressively and its voltage increases so that, when the current crosses zero, the arc is definitively extinguished. The thermal and kinetic effects of the arc plasma during breaking cause a sharp increase of the pressure in the arc extinguishing chamber. The outlet orifices enable the emitted gases to be removed and the pressure to be contained at an acceptable level inside the case of the switchgear apparatus. This removal of the partially ionized breaking gases does however impose minimum safety distances between apparatuses on any one electrical panel in order to prevent any risk of arcing between live adjacent units. It also requires arrangements to be made for overpressures so as not to damage the panel itself. Moreover, the gases outlet from the unit can be considered to be polluting and therefore have to be filtered.
Electrical switchgear apparatuses are moreover known whose case is air-tight so as to eliminate any external manifestation when breaking takes place, as described for example in the document GB 2,119,575. The case then has to be provided with high-performance sealing devices and its mechanical pressure resistance has to be increased. Air-tightness is obtained at the cost of lower breaking performances with respect to a conventional apparatus of the same volume. Furthermore, the cost of such apparatuses as compared to conventional apparatuses is very high, which results in their only being implemented under extreme conditions, for example in explosive environments.
A tightly-sealed low-voltage power circuit breaker is described in the document WO95/08832. This circuit breaker comprises an arc extinguishing chamber arranged facing the contacts and equipped with cooling fins which are arranged between the contacts and an orifice for outlet from the chamber. A recirculation channel directs the gases from the outlet of the chamber to the contact drive mechanism, passing via a de-ionizing filter. The gases emitted when breaking takes place at the level of the arc extinguishing chamber thus flow in a closed circuit in the case and are finally redirected to the contacts and to the inlet to the chamber after they have been cooled and de-ionized. This strong convection enhances high-speed displacement of the arc inside the chamber, which is considered in this document as being particularly advantageous for speeding-up breaking. The arc, as it moves in the chamber, in fact constantly encounters new cold surfaces which perform cooling of the arc. However, the size of the chamber necessary to achieve high breaking performances is thereby increased as the energy absorption capacity of the fins is not fully used due to the rapid progression of the arc. Furthermore, the recirculation channel makes the case more bulky and more complex. Finally, the arc may leave the chamber totally via the outlet opening due to the strong convection. In this case, the arc is no longer in contact with the fins and is no longer cooled.
To better use the heat exchange surface provided by the cooling fins and to prevent the arc from leaving the arc extinguishing chamber, it has been proposed, in the document DE 2,410,049, to house the fins in an enclosure made of gas-generating material, one wall whereof opposite the contacts is provided with gas outlet slits, and to fit a mask comprising holes of small diameter outside the enclosure, the mask being pressed against the wall comprising the slits. The fins are cut into a V-shape so as to form an attack slit. When the circuit breaker opens on a short-circuit current, the arc enters the chamber in centered manner. On reaching the back of the chamber, the arc is deflected onto one or the other of the sides and returns to the contacts diving up between the fins, and is then re-established between the contacts before being projected into the chamber again. The foot of the arc therefore follows a path in the form of a loop until the arcing energy has been dissipated in the fins. The fins are thus used in homogeneous manner. However, the flow of the arc gives rise to periodic restrikings of the arc on the contacts, which damage the contacts.
In the document DE 2,624,957 arc extinguishing plates are described designed for an arc extinguishing chamber of an electrical switchgear apparatus. The plates have a cut forming a tapered U or V-shaped neck extended by a slit opening out onto a circular broadened part. This shape is supposed to ensure a good localization of the arc and fast extinguishing thereof. It is scheduled to cover the extinguishing plates with a plastic or ceramic insulating material, notably polytetrafluorethylene, so as to stabilize the positioning of the arc in the circular broadened part. The plates can be made of soft iron and covered, at the level of the broadened circular part at least, with a soft and electrically insulating magnetic material to prevent the arc from forming a foot on the periphery of the broadened part. According to this teaching, the arc can be localized in the circular broadened part, but nothing is provided to perform extinguishing of the arc. In particular, the arrangements made to stabilize the arc and prevent an arc foot from forming have the consequence of the energy exchange between the arc and its environment being greatly reduced. Apparently the arc can only effectively exchange with the ridges of the plates, at the level of the circular broadened part. The lateral part of the plates is therefore not used efficiently for cooling the arc. Finally, the treatment of the plates in several layers of different materials increases the cost of the device considerably.
The object of the invention is therefore to remedy the shortcomings of the state of the art so as to be able to propose a switchgear apparatus with good performances and greatly reduced external manifestations, in a small volume.
According to the invention, this problem is solved by means of an electrical switchgear apparatus, comprising:
a case defining a longitudinal geometric reference plane and containing:
an opening volume and
an arc extinguishing chamber opening out onto the opening volume and bounded by two opposite side walls parallel to the geometric reference plane, a rear wall located away from the opening volume, a bottom wall and a top wall;
a pair of separable contacts arranged in the opening volume and comprising a first movable contact movable along a flat path in the geometric reference plane between a contact position and a separated position and a second contact;
cooling fins arranged inside the arc extinguishing chamber, perpendicularly to the geometric reference plane, each fin having a free attack edge exposed to the arc;
a bottom longitudinal electrode in electrical connection with the second contact, the bottom electrode at least partially covering the bottom wall of the chamber;
wherein:
the free edges laterally bound a gulley extending in the heightwise direction from the bottom electrode to the top wall, the gulley having:
a first tapered longitudinal end opening out onto the opening volume,
a second broadened longitudinal end forming a stack near the rear wall, the stack having an oblong cross-section in a plane of cross-section parallel to the fins,
a narrow intermediate portion joining the first longitudinal end to the stack, the bottom electrode extending longitudinally in the gulley from the second contact to the stack at least.
The gulley and the stack enable the arc to establish itself quickly and stably at the back of the chamber, in the zone which presents an oblong shape in cross-section. It is known that in an open environment the arc naturally tends to take a general cylindrical shape of circular cross-section. The oblong shape of the stack therefore contributes to a large constriction of the arc, and therefore to a large energy exchange with the fins and the walls at this level. A gas convection current is established between the fins, laterally with respect to the gulley, and enables cooling and de-ionization of the gases to take place in contact with the fins until the arc has been extinguished. This fast cooling of the gases considerably limits the pressure increase in the chamber. The external manifestations are therefore considerably reduced or even totally eliminated. The surface of the fins is used throughout breaking and ensures a very good efficiency in the energy transfer. It should be emphasized that the flat surfaces of the fins are used more to absorb the heat of the gases emitted than to interact directly with the arc. More precisely, the arc does not seem to divide into a multiplicity of arcs drawn in series between adjacent fins.
Preferably, the fins have a given thickness and are separated two by two by a given distance which is of the same order of magnitude as said thickness. Experience shows that the small distance between fins enhances the heat exchange between the gases and the fins, in particular in the phase where the arc is localized in the stack. As an indication, the distance between fins should be comprised between 0.8 and 3 mm, and should be comprised between one half and twice the thickness of the fins. The distance between two fins designates here the smallest distance measured between the two fins, in particular when the fins are not parallel to one another. It should be noted that it is the architecture of the circuit breaker, and in particular the presence of the gulley fostering insertion of the electric arc, which enables the fins to be arranged a short distance from one another. Conventionally in the state of the art, the space between fins is always at least 20% larger than the thickness of the fins, whereas according to the invention considerably smaller distances can be achieved.
Preferably, the stack is limited to the rear by the rear wall. This arrangement favors constriction of the foot of the arc, voltage increase of the arc and pressure limiting in the chamber.
Preferably, the rear wall is not provided with any gas outlet orifices. Flow of the gases between the fins is thus enhanced, thus enhancing use of the whole surface of the fins as heat exchange surface.
According to a particularly advantageous embodiment, the chamber and opening volume together form a closed breaking volume. The arrangement of the chamber enables the pressure increase to be controlled, and therefore enables the chamber to be closed without a major risk of explosion of the apparatus. Alternatively, and for apparatuses of very small volume, it may be preferable to provide at least one calibrated outlet orifice or a pressure relief valve to limit the overpressure in the chamber.
According to one embodiment, the longitudinal bottom electrode is separated from the fins by a distance which is the same order of magnitude as the thickness of the fins. This arrangement enables a large constriction of the foot of the arc to be achieved. Preferably, the lateral volume between the bottom wall and the fins, on at least one of the sides of the bottom electrode, is limited by walls which also contribute to constricting the arc.
Preferably each fin is provided with at least one lateral fixing tab secured in a slit of the side walls, and with at least one rear fixing tab secured in a slit of the rear wall. The rear tab provides a solution to the problem of fixing of the fin and enables the possible mechanical weakness due to the presence of the stack to be compensated.
According to one embodiment, the fins are parallel to one another.
According to one embodiment of the invention, the rear wall is inclined with respect to the fins. This arrangement tends to stabilize the arc at the back of the chamber and to increase the length of the arc.
According to one embodiment of the invention, the gulley extends appreciably in the geometric reference plane, at equal distance from the side walls of the chamber. This arrangement should be preferred for high performances and when the side walls of the chamber have to be prevented from deteriorating too quickly. The gulley in fact constitutes a privileged path for the arc which is thus centered during its displacement to the stack.
According to another alternative, the narrow intermediate part of the gulley is located closer to one of the side walls moving away from the opening volume. This arrangement constitutes an interesting compromise to optimize the dimensions of the chamber. The contacts are located in a mid-plane of the chamber, mid-way between the side walls of the chamber, which enables the opening volume to be optimized. The gulley penetrates obliquely into the chamber. The narrowest fins are used above all for their constricting effect when insertion of the arc takes place. The broadest fins are for their part also used to cool the gases once the arc has established itself in the stack.
Preferably, the switchgear apparatus is a limiting circuit breaker comprising rigid input conductors for conveying current to the contacts, these rigid conductors being shaped in such a way that, when a current flows therethrough, they generate an intense electromagnetic field near the contacts, such as to cause electromagnetic repulsion of the movable contact to the separated position and to project the electric arc into the chamber. Typically, the stationary contact is supported by a U-shaped conductor. Other shapes amply described in the state of the art can however be envisaged. Projection of the arc enables the latter to reach the stack rapidly.